This invention relates to the application of ferroelectric crystals as nonlinear optical media, and more particularly to a method and apparatus for preventing depoling of domains in such crystals from a selected crystallographic axis.
Miniature solid state lasers that produce significant output in the blue and green regions of the visible light spectrum are highly desirable. However, there are technical problems associated with the generation of fundamental frequencies that correspond to such wavelengths using solid state lasers. Consequently, solid state lasers that produce outputs at such wavelengths generally include a means for generating a harmonic of their fundamental frequencies with sufficient efficiency to provide significant output in the blue or green regions of the visible spectrum.
The means for generating a harmonic of the fundamental frequency in solid state lasers is typically a nonlinear optical medium, such as a ferroelectric crystal that has suitable characteristics to provide high conversion efficiency in second harmonic generation (SHG), electro-optic (E-O) modulator and other nonlinear optical (NLO) applications. Potassium niobate (KNbO.sub.3) is one ferroelectric crystal that is ideal for this application because it has a high degree of birefringence in the visible and near infrared regions of the visible spectrum and one of the highest values of nonlinear coefficients for any inorganic crystal. The high birefringence allows the crystal to achieve phase matched conditions in the blue region of the visible spectrum. The high value of nonlinear coefficient provides a correspondingly high degree of conversion efficiency for SHG, E-O modulator and other NLO applications.
Using KNbO.sub.3 as the nonlinear medium in SHG applications, optical conversion efficiencies exceeding 40 percent have been achieved with external resonantly doubled diode lasers, and output powers of greater than 6 mW have been obtained from a diode pumped, internally doubled 946 nm Nd:YAG laser pumped by a 500 mW diode laser. Although the conversion efficiencies of these devices are adequate, certain opto-mechanical properties of KNbO.sub.3 have made its use impractical for most NLO applications.
It is advantageous for any ferroelectric crystal used as a nonlinear optical medium to have its ferroelectric domains poled in a single direction. As-grown crystals almost always have a number of ferroelectric domains. They are poled to a single domain, typically by applying an electric field along the polar axis, or c-axis, of the crystal. Because KNbO.sub.3 has an orthorhombic structure within the range of ordinary ambient temperatures, it has a number of different types of domain, some of which produce noticeable optical distortion at their boundaries. During the poling process, the crystal often becomes noticeably cloudy as its domains are rotated through different orientations to the desired direction.
Furthermore, KNbO.sub.3 crystals are susceptible to depoling after the poling process. Depoling can occur as a result of thermal stress, mechanical shock, or the combination thereof. Depoling typically occurs when the crystal is heated above 70 degrees Celsius. To utilize a KNbO.sub.3 crystal for SHG under noncritical phase matching (NCPM) conditions, with a second harmonic frequency corresponding to 946 nm, it is necessary to operate the crystal at a temperature of approximately 180 degrees Celsius. This almost always causes the crystal to depole as it is cooled back to the ordinary ambient temperature range. It is therefore necessary to minimize the degree of depoling in ferroelectric crystals such as KNbO.sub.3 to successfully use them in practical NLO applications.
One approach that has been attempted in the effort to minimize depoling of ferroelectric crystals during temperature recycling under these conditions has involved the application of a static electric field along the c-axis of the crystal. However, this procedure has not been successfully implemented, and in any case it does nothing to minimize depoling due to mechanical stresses or shock.
One approach that has been used to minimize depoling due to mechanical stress of the crystal mounting has involved padding the crystal with zinc foil. Another approach has involved the attachment of the crystal to a thermoelectric cooler with a flexible, thermally conducting medium. Neither of these approaches is suitable for practical applications in which the mechanical position of the crystal must be rigidly fixed over the life of the laser in which it is used and subjected to a large number of temperature cycles.
Therefore, the practical commercial application of solid state lasers that use KNbO.sub.3 crystals for the nonlinear medium has not been feasible. Instead, efforts have been made to use other less efficient, but more stable, ferroelectric crystals, and to develop other techniques for NLO applications.